In a digital communications system, the carrier frequency is partitioned in time. Each partition represents one symbol. The transmitter modulates each partition with one of the symbols based on the input data to be transmitted. One digital modulation scheme is phase shift keying (PSK). PSK shifts the phase of the carrier to encode either a zero or a one. For instance, the phase of the carrier at the start of the partition is zero degrees if a zero is to be transmitted and the phase is 180 degrees if a one is to be transmitted. The receiver needs to know when the symbols are suppose to change to determine which symbol was sent.
In conventional, digital communication systems the transmitter sends a known pattern of symbols, called a synchronization pattern, before transmitting the data. The synchronization pattern allows a phase lock loop at the receiver to lock onto the phase of the transmitter's carrier. Once the phase lock loop has locked onto the phase it can be used to determine where the symbols change and when the symbol should be sampled. Sampling at the proper time is vital to reducing the bit error rate at the receiver. This system works fine when the data string sent is considerably longer than the synchronization pattern. However, with modern time division multiple access (TDMA) schemes each carrier is divided into multiple time slots. This allows more efficient use of the available spectrum, by allowing several users to access the same carrier. The time slots are short to allow voice communications to be transmitted without any noticeable delay. Using a synchronization pattern to allow a phase lock loop to track the phase of the carrier, requires a significant amount of the slot time, which only leaves a very limited amount time for data to be transmitted during each slot. This can defeat the whole purpose of using TDMA.
One solution to the above problem has been to asynchronously sample the received signal at a rate sufficient to have multiple samples per symbol. This eliminates the need for the synchronization pattern. Each of the sample points is subtracted from a sample point delayed one symbol to obtain a differential phase sample. For a communication system using PSK modulation the differential phase should be either zero or 180 degrees. Noise and inter-symbol interference result in deviations from these ideal phases. By adding up the deviations for each of the possible sample points over a substantial portion of the time slot, it is possible to determine which of the sample points has the minimum deviation. This sample point is chosen to sample the symbols. However if the timing of the receiver's clock or transmitter's clock is off this will show up as a drift that cannot be corrected using the above Scheme. This will tend to result in more errors at the beginning and end of the slot.
Thus there exists a need for a method and apparatus that does not require a synchronization pattern and can adjust for drift in the receiver's or transmitter's clock.